The knowledge of deep sea currents is important in oil-well boring techniques out at sea, especially for calculating the stresses on the immersed structures. Furthermore, in the case of action by divers or submarines, it is important to know the speeds of the currents for safe working. Another application is the search for nodules.
It is known to measure the speed and the direction of currents by means of a propeller current meter fixed to the sea bed by an anchoring line.
The drawbacks of this device are in only providing a pin-point measurement, of limited accuracy because of the mechanical friction of the propeller and at a depth limited because of the difficulty in providing too long an anchoring line.
It is further known to measure the currents by means of a Doppler effect sonar. The signals used are obtained by volume reverberation of the acoustic waves. These devices supply the speeds of the currents in different depth ranges with good accuracy but they have the drawback of not being able to be used at depths greater than 1000 meters because of the absorption of the acoustic waves and the divergence of the beam emitted by the sonar.